Most backfiring issues are the result of an incorrect air/fuel mixture. However, if it is only happening when the engine is turned off, it sounds more like a dieseling issue.
What I would suggest is to first check if there are any diagnostic trouble codes stored in the computer. This vehicle has an OBD1 system which doesn't require a scan tool to retrieve codes.
Here is a link that explains how it's done:
Next, here is a link that is focused on repairing a backfire, but primarily when the engine is running. But, it may provide some helpful insight.
If this is happening when the key is off, there could be excessive carbon on the pistons which simply gets hot enough to ignite the fuel still in the system. If it is coming through the intake, you may have an intake valve that is failing.
Let me know how it runs when the engine is running, and if possible, record what it does and upload it for me to hear.
Thursday, July 28th, 2022 AT 1:47 PM